This study was undertaken to determine the extent and severity of respiratory disease caused by CAR bacillus in immunodeficient mouse strains. Mice were inoculated intranasally with a standard number of CAR bacilli. These mice were killed at 2, 4, 8, and 16 weeks post inoculation. Tissues were obtained to determine the extent and severity of lesions by histopathological examination. The in-vivo portion of the pathogenesis study has been completed and histopathologic analyses demonstrated that pulmonary lesions were most severe in BALB/c, comparable in severity in the CB17 scid and nu/nu, and least severe in the bg-nu-xid. An additional study using inoculated and naive BALB/c mice was completed which demonstrated the ability to transmit the agent via an aerosol route. The initial transmission study has been completed and histopathologic analyses are in progress.